If you boil water over a camp fire on the sea shore, you have to heat it to two hundred and twelve degrees. But on a mountain top, water boils before it gets as hot as that. This is because, on low ground, there is more air above the water than on land a mile or two higher. The lighter the air pressure the easier it is for water to expand into gas. Therefore, it takes less heat on a high mountain to make water boil. Of course, then, boiling water away up on the Alps isn't nearly as hot as boiling water on a sea beach. In some very high places boiling water should be just about right for a warm bath, and water there would escape in gas long before it was hot enough to boil an egg.
